Marijuana is the most widely used illicit drug in the United States. It impairs equilibrium and is often found in the bloodstream of drivers involved in automobile accidents. While the effects of marijuana on equilibrium and driving have both been studied separately, a critical gap in the existing knowledge of marijuana effects on driving is the lack of a rigorous examination of these effects within the same group of subjects. The specific aim of the proposed study is to compare the effects of placebo and two potencies of marijuana on equilibrium and simulated automobile driving using a within-subject design. Twenty adult paid volunteer marijuana users with driving experience will undergo a battery of six tests at potencies of 0, 1.77, and 3.95% delta-9 THC via smoked cigarettes. The tests will include a) brake reaction time within a driving simulator, b) risk-taking maneuvers within the driving simulator, c) the EquiTest balance measurer, d) the critical flicker fusion test of CNS activity, e) a sensorimotor choice reaction time task, and f) subjective effects as measured by visual analog scales. Results will show whether marijuana doses that impair driving also affect motor equilibrium and vice versa. Marijuana doses that objectively appear to increase "cautious" driving may simultaneously worsen other essential aspects of driving, such as sense of balance. The varied tasks and within-subject design of the proposed research will clarify the various effects marijuana has simultaneously within a driver.